The present invention relates to helical screw compressors. More particularly, the present invention relates to a multi-rotor screw compressor having three or more rotors.
Multi-rotor screw type compressors are typically used to compress various working fluids for air conditioning and refrigeration applications. Multi-rotor compressors generally include a housing to enclose and protect the interior components of the compressor (such as the rotors). In a multi-rotor compressor, the rotors usually include a lobed sun rotor that intermeshes with, and typically drives, multiple adjacent lobed planet rotors. The intermeshed sun rotor and each adjacent planet rotor act as a compression pair; rotating about their axes relative to the housing to move the working fluid from suction inlet ports at a low pressure to discharge outlet ports at a higher pressure. The compression of the working fluid takes place in the spaces between and adjacent the flutes and lobes of the sun and planet rotors and the housing. These spaces are typically referred to as compression pockets. Each compression pocket receives working fluid as the pocket rotates with the rotors to open to a portion of the suction port. Each compression pair is also in communication with a portion of at least one discharge port. Working fluid within each compression pocket rotates with the rotors and is discharged as the rotors align with the discharge ports.
After flowing through the discharge ports, the working fluid enters a discharge channel, which interconnects with a piping system to transfer the working fluid to other components in the air conditioning or refrigeration system. It is desirable to achieve an internal pressure of the working fluid in each compression pocket equal to the pressure in the discharge channel at the moment just before each compression pocket opens to the discharge port. If the internal pressure at this moment differs from the pressure in the discharge channel, a rapid flow of working fluid through the discharge port occurs each time each compression pocket opens. This rapid flow of working fluid allows the internal pressure and the discharge channel pressure to become equalized. The flow velocity of the working fluid through the ports during this short moment of equalization is often much higher than the flow velocity of the working fluid when it is displaced out through the discharge ports by the rotors. This increase in fluid flow velocity, (and associated pressure pulsations) causes noise that may be disturbing to individuals located near the system, and may cause pressure pulsations and vibrations in various other system components that may damage the system components. The pressure pulsations may also decrease the efficiency of the compressor. It is often difficult to adapt the internal pressure to be equal to the discharge channel pressure. This is because the difference between the discharge channel pressure and the internal pressure at the end of compression process may vary as a result of many factors including: outside ambient conditions (including temperature and humidity), condenser size, and the cooling ability of the cooling medium used at the condenser.
Similarly, a change in suction flow rate may also cause suction pressure pulsations and fluid flow surges in the suction channel upstream of the suction ports. These pulsations may result in undesirable noise and vibration, and may also detrimentally affect system operating efficiency.
Typically, multi-screw compressor designs include multiple suction ports and discharge ports which correspond to and communicate with the multiple compression pairs. The geometry (the size, shape, and disposition) of each of the multiple suction ports is identical. Likewise, the geometry of each of the discharge ports is also usually identical. The identical geometry of the ports, coupled with the fact that the planet rotors are also usually of an identical size and helical geometry, and are rotated at the same angular velocity in a sun driven multi-rotor compressor, exposes or “opens” the working fluid from each compression pocket to a portion of the ports at the same time. Similarly, each compression pocket “opens” and “closes” to a portion of the suction ports at the same time. This identical porting is due to the symmetrical geometry of the suction ports with respect to the compression pairs, and the equivalent angular velocity of the planet rotors driven by the common sun rotor.
Thus, in a typical multi-rotor compressor the simultaneous opening and closing of multiple compression pockets has the undesirable effect of increasing the flow velocity of working fluid to and from the channels, as the internal pressure of several compression pockets open simultaneously to the channel and must be equalized with the pressure in the channel. Thus, when multiple compression pockets open simultaneously (in-phase with each other), the peak amplitude of the pressure pulsations in the channels increases.